Acrobat Reader won't open epubs, even good ones. Sigil needs to be set up so it will correct errors on input. Calibre editor doesn't require a valid epub. If you are desperate you could just paste your book sections into a new blank book in calibre editor and then use tools, table of contents to generate a valid table of contents. It, as does Sigil assumes you have marked your chapters with headings. <h1><h2><h3> etc.
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